White-Noise Generator

By:
Brad Graham, inventor, electronics hobbyist, and author

Topic

Skill Level

Physics

White noise is a smooth hiss consisting of all audio frequencies played back at random simultaneously. Because your ears cannot "lock" onto any specific frequencies or patterns in white noise, this device has the effect of shutting off your ears to distracting noises. White-noise generators have many uses, some of which include the testing of audio equipment, eliciting a calming effect on humans, and even medical uses.

This project is so simple that you might think there is something missing, but the unit you will build works very well, and exploits the fact that transistors generate random noise when reverse-biased. Try out your completed project as a sleep aid in your noisy apartment, as a means to secure your next hacker gathering to defeat eavesdroppers, or even as a dual-channel stereo system.

Tools

Soldering equipment

Dull blade for stripping wires

BE CAREFUL! Soldering irons and solder can get very hot and cause serious burns. The electric drill can be dangerous, too. Always use caution when using any tool that can burn or cut you. Adults should supervise children's use of tools.

Assembly

Use the white-noise generator schematic shown in
Fig. 1 as your starting point.

Fig. 1. White-noise generator schematic.

1. Breadboard the circuitry

The simple circuit can be built on a solderless breadboard (see Fig. 2) in a few minutes. First, feed the reverse-biased emitter-to-base junction of Q1 into a 1-Ω audio amplifier integrated circuit (LM386) so that it can power a speaker or a set of headphones. Because many transistors will act differently in this application, add a 100K variable resistor (VR1) to allow for some “tweaking” of the white noise for best results. You can use practically any small-signal NPN transistor in this circuit. The commonly available 2N3904 gives great results. Another variable resistor (VR2) controls the level of noise reaching the audio amplifier so that you can adjust the level to suit your needs. Even though the LM386 audio amplifier is just a small, 8-pin integrated circuit (IC), it actually can drive any size speaker you like, and is probably more than loud enough to fill a room full of clean white noise.

Fig. 2. Breadboarding the white-noise generator.

For a very relaxing white-noise generator that will seem to send noise in all directions, you can build the stereo version shown in Fig. 3 simply by doubling up the original version, and sending the outputs to a pair of stereo headphones, speakers, or the line inputs on your audio amplifier. Because there are two independent noise generators running, the sound will seem to come from all directions, which works amazingly well for audio-deprivation experiments where the subject is wearing headphones or is placed directly between the two speakers.

Fig. 3. Stereo white-noise generator schematic.

If you do plan to build the stereo version, try to keep all parts the same make and value, so that the sound is even on both channels. Because some transistors may behave slightly differently in this circuit, they should both be exactly the same. To simplify the stereo circuit a bit further, you could replace the LM386 with a stereo amplifier IC to reduce the parts count a bit. If you plan on feeding the output directly to a line input on an amplifier, then you actually can eliminate the entire amplifier section and simply feed the output from the center tap of VR2 directly to your amplifier’s input.

2. Power up and adjust the system

As soon as you power up the circuit, a nice clean hiss should be heard from the speaker, if everything is working well. If your noise sounds more like a crackling or spitting oscillator, then try moving VR1 to tweak the sound a little bit. If your noise is still not smooth and crackle-free, just drop in another transistor, and listen to what you get. The optimal white noise is a very clean hiss much like a radio with no station or the sound of air leaking out of a tire. Since the LM386 audio amplifier is happy on any voltage from 9 to 18 V, you might want to try a 9-V battery for the absolute cleanest possible output. A dirty power supply will add hum into the circuit.

If you are planning to use headphones, turn down the volume control (VR2) at first, because the output of the audio amp can be very loud. If you find the output to be much too loud, remove capacitor C2, and that will change the gain on the LM386 from 200 to only 20, making the output much quieter.

If you are happy with the “flavor” of white noise after adjusting VR1, then you simply can measure the impedance when it is set correctly and replace it with a fixed resistor to simplify the circuit even further. You may decide to leave the tweaking adjustment because it works somewhat like a single-band equalizer to adjust the sharpness of the white noise.

3. Transfer and install the system

Now you can move the circuit from the breadboard to a more suitable performance (perf) board. The perf-board version of the white-noise generator shown in Fig. 4 is extremely small, so it can be installed into just about any cabinet.

Fig. 4. White-noise generator on a perf board.

On a single 9-V battery, the white-noise generator will run all day, although you also could use a 9- or 12-V direct-current (dc) adapter for batteryless operation if needed. The 4-inch-diameter speaker shown in Fig. 4 may be fine for a small room, but for a larger room or a more bassy sound, you probably would want to use a larger speaker or one already mounted inside a cabinet. A 10-in speaker in a cabinet will make the white noise much richer and more like wind rather than a hissing radio.

4. Generate some noise!

Fig. 5. Head-mounted stereo version.

The head-mounted stereo version of the white-noise generator shown in Fig. 5 is great for relaxing or messing around with auditory-deprivation experiments. This version has dual noise generators, and both audio amplifiers have capacitor C2 removed so that the output is not too loud for the headphones. To connect this unit to a stereo amplifier for big-room sound, you just need a 1⁄8-inch stereo-to-RCA-jack adapter so that it can be fed into the line input of any stereo system.

Fig. 6. A larger version with more bass response.

The cabinet-mounted system shown in Fig. 6 is great for filling a room with rich, bassy white noise that sounds more natural than what comes from a smaller speaker. Because of the full range the quality speaker allows, all the tones are represented rather than just the higher, “hissy” frequencies.

Fig. 7 shows what white noise looks like on an oscilloscope or computer input. Because white noise is comprised of every possible frequency at ;the same time, there is no discernible pattern or tone that can be detected, which is why your mind finds it so relaxing to listen to.

Fig. 7. White noise is a random wave.

White-noise generators are often used in digital machines that need to create some kind of chaos for a random output. Digital encryption and lottery machines are prime examples of using white noise as a source for random information, because digital circuits cannot generate truly random sequences that never repeat.

The white-noise generator you've built here is great for use as a sleeping aid or when you want to work in a noisy environment without being distracted by background noise. Some ideas for design might be the addition of a voltage sweep circuit to simulate ocean waves, or perhaps the addition of some type of audio equalizer or filter to shape the white noise into pink or brown noise.